Add Cloud Console cancel action and cancelled status

- Widen TaskStatus to include "cancelled"; add it to TasksView's
  STATUSES filter dropdown.
- Add TaskCancellationResponse type and cancelTask(taskId) to api.ts.
- Add a Cancel button to TasksView's task detail panel, gated on
  tasks:submit and a non-terminal task status; updates the displayed
  status on success and surfaces errors via the existing error path.
- Extract the cancellability rule into a pure taskCancellation.ts
  module (mirroring taskProgress.ts/plannerHistory.ts) with unit
  tests, since the project has no Vue component-mounting test setup.

Task 6/9 of task-cancellation change.

cloud-console/src/api.ts

@@ -13,6 +13,7 @@ import type {
   PluginRecord,
   PluginRegistrationPayload,
   TaskAttempt,
+  TaskCancellationResponse,
   TaskListResponse,
   TaskSubmissionPayload,
   TaskStatus,
@@ -239,6 +240,13 @@ export function getTaskAttempts(taskId: string): Promise<TaskAttempt[]> {
   return request<TaskAttempt[]>(`/v1/tasks/${encodeURIComponent(taskId)}/attempts`);
 }
 
+export function cancelTask(taskId: string): Promise<TaskCancellationResponse> {
+  return request<TaskCancellationResponse>(
+    `/v1/tasks/${encodeURIComponent(taskId)}/cancel`,
+    { method: "POST" },
+  );
+}
+
 export function getTaskPlannerDecisions(
   taskId: string,
   attempt: number,